1. Field of the Invention
The invention relates generally to a device which measures temperature of an object, such as a thermometer. More particularly, the invention relates to a system and method for measuring temperature using an infrared detector, without having contact with the object.
2. Description of the Related Art
I. Infrared Thermometers
Infrared (IR) thermometers are devices which measure temperature without having physical contact with the object being measured (the "object"). The IR thermometer detects intensity of IR radiation which is naturally emanated from the object's surface. For objects having temperatures in the range of 0.degree.-100.degree. C., the IR thermometer uses an IR sensor which detects IR radiation having wavelengths between 3 and up to approximately 40 micrometers (.mu.m). IR radiation having such wavelengths is commonly referred to as "thermal" radiation. An example of such an IR thermometer includes a medical ear thermometer which measures temperature from the tympanic membrane and surrounding tissues of an ear canal without contact.
The IR thermometer typically includes a housing which may have a variety of shapes depending on its application. It is desirable that the housing have a probe containing a thermal IR sensor which receives and detects IR radiation from the object. In medical applications, the probe typically has a shape suitable for insertion into an ear canal. Moreover, the IR thermometer may include some additional features which may aid in inserting the probe into the ear canal. For example, U.S. Pat. No. 4,993,419 issued to Pompei et al. discloses such additional features.
II. Infrared Sensors
Typically, a thermal IR sensor comprises a housing having a transparent IR window at its front end, and at least one sensing element therein. The sensing element responds to thermal radiation energy (i.e., IR flux) which enters the sensor through the IR window. The sensing element is characterized by a thermal time constant which is directly proportional (by a factor 1/12-1/10) to the time needed for the sensing element to reach a steady-state level. The IR sensor produces an electric response representing the net IR flux existing between the sensing element and the object. Using appropriate data processing techniques, the electrical response may be related to the object's temperature. Typically, at least two types of data are used in calculating temperature of the object's surface. The first data includes the magnitude of the IR flux, and the second data includes a reference temperature. The reference temperature is measured using one of several conventional contact temperature detectors, such as a semiconductor, thermistor, or thermoelectric detector. To measure surface temperature of the object, the IR thermometer detects and converts IR radiation into electrical signals suitable for processing by conventional electronic circuits.
There are several types of IR sensors which may be used to accomplish this task. These IR sensors include quantum and thermal detectors. A brief description of each of these IR sensors is provided below.
The IR sensor used in measuring the net IR flux requires a special opto-electronic detector which is responsive to thermal radiation flux (e.g., IR flux). IR radiation has electromagnetic properties and, thus, may be detected using opto-electronic type detectors, such as a quantum detector or thermal detector. The quantum detector, such as a photoresistor, requires cryogenic cooling since when operating near room temperatures, it generates an unacceptably high noise level. On the other hand, a thermal detector may operate at normal room temperature without the need for cooling devices. Thermal detectors include thermopiles, pyroelectrics, bolometers, and active far infrared (AFIR) detectors.
Some IR thermometers employ focusing optics for narrowing their fields of view. This may be required when temperature is measured from remote objects. On the other hand, IR thermometers for measuring temperature from a cavity, such as a human ear, produce better results with a wide field of view. A wide angle improves the accuracy of measurement, and makes the measurement less dependent on the operator's technique. For further information on such wide-field IR thermometers, reference is made to U.S. Pat. No. 4,797,840 issued to Fraden, and U.S. Pat. No. 5,368,038 issued to Fraden.
The bolometer IR sensor is a thermistor-based IR sensor which includes a temperature sensitive resistor. Due to its relatively large thermal mass (as compared with thermopiles), a bolometer is slow in detecting IR radiation, unless fabricated on thin membranes. To improve its response speed, the bolometer is sometimes made to be very small in size and is supported by tiny wires. However, this configuration makes it impractical for medical applications. Other bolometers are made as film bolometers. An example of a film bolometer is disclosed in U.S. Pat. No. 4,544,441 issued to Hartmann et al. However, the film bolometer IR sensor is comparatively costly and suffers from poor sensitivity, nonlinearity, poor manufacturer's tolerances, and drifts. Thus, film bolometer sensors are seldom employed in IR thermometers.
IR thermometers having a bolometer sensor suffer from two major drawbacks: a slow speed of response, and a high sensitivity of the output signal to thermistor tolerances. For example, to achieve a reasonable accuracy in a medical infrared thermometer, it is desirable that the thermistor bolometers that are connected in a bridge circuit maintain a mutual stability over the lifetime of the device. This stability is of the order of 10 ppm or 0.001%--an extremely tight tolerance which is not readily achievable at present state of technology. Prior efforts have failed to teach how these sensors may be implemented to achieve fast response and maintain high sensitivity, without resorting to complex and expensive designs. For more information on these efforts, reference is made to U.S. Pat. No. 3,581,570 issued to Wortz, U.S. Pat. No. 3,282,106 issued to Barnes, U.S. Pat. No. 2,865,202 issued to Bennet, and U.S. Pat. No. 3,023,398 issued to Sieget.
III Accuracy
Present IR thermometers suffer from disturbances and interferences. Interferences may include, among other things, mechanical and electrical noise, aging of components, thermal drifts, manufacturer's tolerances, nonlinearity, and effects of ambient temperature. One way of compensating for additive noise has included employing a dual sensor. A dual sensor comprises a sensor having two sensing elements which are connected in a bridge circuit. Employing a dual sensor may reduce errors from interferences which influence both sensing elements in an identical manner. For interferences which affect both elements in a different manner, canceling drifts may be achieved by subtracting an IR reference signal from the detected signal. This technique, however, is less effective when applied with the bolometer sensor. This is true since the output signal from the bolometer sensor includes a high bias component which depends on ambient temperature and which may not be effectively canceled by the reference technique. For more information on this technique, reference is made to U.S. Pat. No. 4,602,642 issued to O'Hara et al. and U.S. Pat. No. 5,169,235 issued to Tominaga et al.
IV Response Speed
In thermopiles and some bolometers, a very thin-membrane detector having a thickness of about 1 .mu.m is used to achieve a fast response. The use of a thin membrane sensor results in higher cost and limits the size of a detector. In pyroelectric sensors, the speed is achieved by measuring the rate of temperature change instead of actual temperature, and this requires use of a mechanical shutter, thereby adding more hardware and complexity. In AFIR sensors, a servo-loop is used to achieve a fast response. However, the use of a servo-loop introduces more noise, potential sensor instability, and calibration difficulties.
Except for pyroelectric sensors, IR thermometers rely on achieving a steady state level of the output signal following exposure of the sensor to the net IR radiation. In fast sensors, such as thermopiles and AFIRs, this level is reached within 0.2-1.0 second following exposure of the sensor to a thermal step function. In slow sensors, like the bulk thermistor bolometers, tens of seconds and even minutes may be needed to achieve a steady-state level. The need for reaching a steady-state level is not unique to IR sensors, but is also common to conventional contact thermometers. For instance, a mercury-in-glass thermometer typically takes about 3 minutes to measure human temperature orally, because the patient has to wait until the temperature of the mercury bulb reaches that of the sublingual cavity. Contact thermometers which require a steady-state level to be reached are commonly referred to as "equilibrium" thermometers. As used herein, the term "equilibrium" refers to substantially near equality, and not necessarily exact equality, of temperatures of the object and the sensor. Also, the term "conventional" thermometry refers to "contact," non-IR thermometry.
In conventional thermometry, a predictive technique is applied to quickly achieve results from a slow response sensor. In general, a predictive technique relies on an algorithmic data processing of output signals from a conventional sensor. Some predictive techniques rely on software-based data processing, and others on hardware-based implementations. In using predictive techniques, the equilibrium level between the sensor and the object is not achieved, but is, rather, mathematically computed (i.e., predicted) based upon more than one sample of a changing sensor's response. Hence, the temperature measurement and display of the predicted temperature may be accomplished before equilibrium is reached. For more information on this technique, reference is made to U.S. Pat. No. 3,978,325 issued to Goldstein et al., and U.S. Pat. No. 3,872,726 issued to Kauffeld et al.
In IR thermometry, however, the predictive technique of conventional thermometry is very difficult and, in many cases, impossible to apply due to several factors. One factor is due to the inability of the IR sensor to reach the object's temperature. Even for a very hot object, the temperature of the sensing element of an IR sensor may still differ from the sensor's housing temperature by a few tenths of a degree, no matter how long one may wait. Therefore, conventional predictive algorithms may not predict the correct temperature of the object. Another factor is the inaccuracy of the predictive technique in the presence of noise. To achieve a reasonably high signal-to-noise ratio (SNR), the sampling period of the signal (and, subsequently, the time of prediction) should be close to a thermal time constant of a sensing element and, preferably, longer. For example, if the thermal time constant of a bolometer is 5 seconds, the equilibrium response would be about 1 minute and the predictive response is 5 seconds or longer. However, both of those times are too long for many applications. Considering the increased error and longer measurement times required to achieve acceptable results, a predictive technique does not lend itself to use in IR thermometry.
Therefore, there is a need in temperature measurement technology for a system and method which achieves a fast and accurate response using IR thermometry. Such a system should be easy to implement and cost effective in medical and veterinary applications, for example.